In recent years, full color digital copying machines, serving as the full-color image forming apparatuses, have become widely prevalent which form full color images, based on image data of red (R), green (G) and blue (B) colors, obtained from color original documents. In order to obtain stable outputted images to satisfy the users, stabilizing controls, color-registration corrections and toner-density corrections are executed by said full-color image forming apparatuses, while said full-color image forming apparatuses do not conduct the image forming operation itself, or while said full-color image forming apparatus conducts the image forming operation as a usual copying operation. For example, concerning toner-density correction, which is conducted during image forming operation, a correction patch of each color is formed on a portion between successive transfer sheets, so that the toner-density correction can be successfully conducted, while the full productivity of said full-color image forming apparatuses is unchangeably maintained.
Unexamined Japanese Patent Application Publication No. 2000-35703 (page 6, FIG. 5) discloses an image forming apparatus in which a registration pattern for the registration correction is formed between two toner images formed on a recording member, being a first timing, and further the shifted amount of the registration of said formed registration pattern is detected at a second timing, which is different from the first timing, so that the targeted registration correction is conducted. According to said image forming apparatus, the formation and detection of the registration pattern and the registration correction are conducted at specific timings, corresponding to different sheet intervals on the image carrier, so that the interrupting time during image formation can be reduced.